1. Field of the Invention
This invention is directed toward the field of spectral signature sensors useful for target recognition and more particularly, the invention provides a method and an apparatus for performing spectral recordation of images for the purpose of identification, matching or storage.
2. Prior Art
Spectrophotometers (color meters) are widely used in the area of target recognition. The principle of recognition is that different targets reflect, emit or absorb light differently. Alternatively, different targets represent independent light sources, each target having an observable spectra which is particular to that target. Baird, in U.S. Pat. No. 3,343,448 describes a spectroscopic apparatus useful for analyzing the emission spectrum of an object positioned along the optical axis of a reflecting zone plate. Baird's apparatus has means for excitation of a sample located along the optical axis of the apparatus. The sample, once excited, emits light having a characteristic sample-specific spectrum which is gathered and collimated by a reflecting lens and brought to focus on the reflecting zone plate. The reflecting zone plate is analogous to a diffraction grating in which dispersion of reflected light occurs along the optic axis. The emission spectrum emanating from the sample is reflected from the zone plate and brought to focus on a photodetector, which also lies along the optic axis and which is capable of motion with respect to the reflecting zone plate. The various colors comprising the emission spectrum are brought to focus at different points along the optic axis, depending upon the color. Thus, the position of the translating detector with respect to the reflecting zone plate together with the signal out of the photodetector provides a measure of the emission spectrum of the sample.
One difficulty with employing the Baird-type of apparatus to target recognition, in general, is that to avoid light loss the target must lie along the optic axis defined by the reflective zone plate and concave mirror. More particularly, since the Baird apparatus utilizes reflective optical elements, the source or target must lie between the reflecting mirror and the reflecting zone plate. Such an optical construction is not operable for analyzing the spectrum of light emanating from a remote object or target or a group of such targets within a field of interest.
U.S. Pat. No. 4,742,222 to Retfalvy et. al. discloses a device which employs the longitudinal chromatic aberration of a dispersive lens to separate light from a single "point-like" source (having a narrow predetermined wavelength range and angled field of vision) from the background signals due to other light sources that have a different wavelength range and/or a different field of view. In this device, an aperture is placed between the light collection system and a detector. The longitudinal distance between the optical collector and the aperture surface, together with the aperture size determines the center wavelength and spectral bandwidth, respectively, of the predetermined wavelength range. The lateral displacement of the aperture from the optical axis determines the field of vision for the system. The device is operable only for detecting a single point-like source that emits light in a narrow wavelength range. Further, there is no means presented for processing the image as, for example, to identify the source.
In U.S. Pat. No. 4,705,396 to Bergstrom, Bergstrom describes an image monochromator arranged to receive light at an input simultaneously from an entire object field to be viewed and to produce a viewable image of the entire object field simultaneously at a selected wavelength band. The apparatus combines an imaging lens and a monochromator from the prior art to obtain images in different wavelength ranges. The purpose of the diffraction grating (diffractive element) is to displace the images formed in the different wavelengths in a lateral direction, i.e., the displacement lies in a plane that is perpendicular to the optical axis. By itself, the (prior art) imaging lens can form an image, but cannot form distinct images in different wavelength ranges. On the other hand, the (prior art) monochromator can only distinguish the spectral composition of the point-like object, located at a specific field of view. The invention combines these prior art elements to provide a means for forming distinct wide-field images in different spectral ranges.
A color signature sensor for analyzing the spectrum of light reflected from the surface of a remote object is described in U.S. Pat. No. 4,954,972 to Sullivan. This apparatus employs a lamp to irradiate the object and a pick up fiber optic to conduct the light reflected from the object to a diffraction grating. The diffraction grating receives the light from the fiber optic and disperses it, bringing it to focus on a substantially linear detector array. The detector array comprises individual photosensitive pixels which are sampled and analyzed to reconstruct the spectral image of the object.
Image multi-spectral sensing (IMSS) differs from the foregoing devices in that it records the spectrum of individual targets within an image or scene. It is capable of simultaneously recording the spectrum of many different points or targets within an image or a field of view. It is desirable to have an apparatus which is compact and is suitable for simultaneously analyzing the spectral composition of light emanating from one or more targets within an image.